The present invention relates generally to methods and machines for manufacturing composite tubular articles such as golf club shafts and the like.
Composite articles comprise reinforcing fibers embedded in a matrix. Graphite, boron, glass, KEVLAR and other fibers are well-known. Epoxy is a well-known matrix. The fibers may be aggregated in woven textiles or in strips or sheets in which the fibers are uniaxial, i.e., unidirectionally aligned. Strips impregnated with epoxy resin are commonly known as "pre-preg" strips. To manufacture a composite article, pre-preg strips are laid upon or wrapped around a form or bonding tool in multiple plies. Although, to a certain extent, the somewhat tacky resin adheres the pre-preg strips to the bonding tool, the strips quickly begin to separate from the bonding tool, particularly if the tool is non-planar or contoured. Separation of the pre-preg from the bonding tool will not only result in a composite article that is aesthetically undesirable because it does not correspond to the contours of the bonding tool, but will also result in structural defects in the article. Adhesion can be facilitated by pre-heating the bonding tool, but to maintain the pre-preg strips firmly in contact with the surface of the bonding tool during subsequent curing processes the assembly is typically tightly enclosed in a wrapping material, such as cellophane film or a plastic bag. The wrapped assembly is then cured at a high temperature, typically between 180 and 350 degrees Fahrenheit. If the wrapping material by itself would not otherwise adequately secure the pre-preg in contact with the bonding tool during curing, the assembly may be cured in an autoclave at both a high temperature and a high pressure. The pressure collapses the bag-like wrapper and thus presses the pre-preg firmly against the surface of the bonding tool. When curing is complete, the wrapper is removed from the assembly, and the resulting composite article is removed from the bonding tool.
Golf club shafts, fishing rods and other tubular articles may be fabricated using composite materials. To fabricate such tubular articles, the pre-preg strips may be wrapped around a cylindrical bonding tool or mandrel and then cured as described above. As with any composite manufacturing process, it is desirable to ensure that the pre-preg adheres to the mandrel and conforms precisely to its taper. Rolling machines are known that comprise two generally parallel planar surfaces or platens, between which the mandrel is rolled while wrapping it with pre-preg strips. The angle between the platens may be adjusted off-parallel to conform to the mandrel taper. A drive mechanism moves the lower platen with respect to the upper platen and thereby rolls a mandrel sandwiched between them. Pre-preg strips fed between the rolling mandrel and one of the platens will thus generally be securely wrapped around the mandrel if the mandrel has a uniform taper. The mandrel may be pre-heated to facilitate adhesion of the pre-preg.
Although the above-described process is adequate for fabricating tubular composite articles having uniform tapers, the planar platens cannot roll pre-preg strips tightly against the surface of a mandrel having a non-uniform taper or other complex contours. The platens of rolling machines may therefore be provided with a resilient surface such as a rubber pad. Nevertheless, the rubber pads do not exert a sufficiently uniform pressure against the mandrel; more pressure is exerted on sections of the mandrel having greater diameters than is exerted on sections of the mandrel having smaller diameters. To further increase pressure uniformity, the platens may be divided into narrow sections that can be likened to the keys of a piano. Each piano key-like section is resiliently cushioned independently of the other sections. Nevertheless, a uniform pressure cannot be exerted against a mandrel having a diameter that changes over a distance less than the width of a single key-like section.
It would be desirable to provide an apparatus for rolling pre-preg around a mandrel having complex contours that maximizes pressure uniformity. These problems and deficiencies are clearly felt in the art and are solved by the present invention in the manner described below.